Snakes don’t just live on land; some live in the ocean too. But a new study has revealed that sea snakes are vanishing from a coral reef in Australia — and it’s not clear why.

The team surveyed Ashmore Reef in the Timor Sea, a former hotspot for sea snakes. In 1994, researchers estimated that nearly 40,000 sea snakes lived in a 174-square-kilometer patch of the reef. And the reef would seem to be an ideal habitat, since it was declared a protected nature reserve in 1983.

The researchers compared sea snake observations made in the 1970s and 1990s to the results of new surveys in 2002 to 2010. In 1973 and 1994, scientists reported seeing 42-46 snakes per day; that dropped to 1-7 in 2005-2010. Researchers had previously spotted nine species in about a week in earlier surveys; in the most recent three surveys, the team saw only two.

The study authors mull over several possible explanations, including loss of coral cover, a drop in prey numbers, and diseases brought in by illegal boats, but none seem entirely satisfactory. “[A]lthough the causes for the declines are not known,” the team writes in Biological Conservation, the reef’s protected status “has not prevented their occurrence.” — Roberta Kwok | 13 August 2013